Although the use of human stem cells for the generation of pancreatic β cells, and thus the treatment of a large population of T1D and T2D patients, is an exciting prospect, numerous hurdles need to be overcome to ensure the efficiency of the differentiation process and the safety of the final product. For example, undifferentiated ESCs form teratomas on transplantation into immunocompromised animals, which have been observed on transplantation of hESC-derived pancreas progenitors cultured for ~12 d in vitro (Kroon et al. 2008
). Interestingly, when hESCs were differentiated under similar conditions for an extended period of time (20 d) (Jiang et al. 2007
), no teratoma formation was observed in host animals, suggesting a greater degree of differentiated cells and loss for neoplastic transformation. Thus, the risk for teratoma formation can be reduced either through elimination of undifferentiated cells via purification methods or through efficient promotion of differentiation.
Another critical aspect centers on the complement of different cell types present at the final differentiation stage. For example, transplantation of pancreas progenitor cells also results in the development of exocrine pancreas structures, e.g., acinar and duct cells, albeit at much reduced frequency compared to endocrine cells (Kroon et al. 2008
). In addition, other endocrine cell types, including glucagon-producing α cells, are likely to be generated as well. Although the entire complement of endocrine cell types similar to that found in human islets is likely beneficial for full function of these structures, the presence of exocrine acinar cells that might release enzymes in an uncontrolled manner is worrisome. Furthermore, as described above, acinar cells can transiently or permanently differentiate into cells with progenitorlike activity under conditions of injury or inflammation, and even develop into neoplastic lesions in the presence of oncogenic mutations (Morris et al. 2010
). Although the likelihood of such scenarios is small, extensive tests need to be performed to ensure that nonendocrine cells do not compromise the function of the hESC-derived endocrine cells or pose cancer-related risks.
Other unresolved issues include the immune response that will be directed against hESC-derived cells on transplantation into immune-competent T2D and autoimmune T1D individuals. As addressed in detail elsewhere in this collection, immunosuppressive regimens have been developed over the last few years that now provide significant and long-lasting protection against cadaveric islets that are transplanted into diabetic patients (Posselt et al. 2010a
). In addition, improved encapsulation devices are currently being developed to not only shelter hESC-derived endocrine cells from the immune insult of the host individual (Vaithilingam et al. 2008
), but also prevent escape of potentially tumorigenic cells into the host body.
Finally, it will be critical to ascertain that the hESC/iPSC-derived β cells are truly equivalent to the endogenous counterparts. β cells are highly specialized cells that not only produce and secrete insulin, but do so in a tightly controlled manner. Critical aspects of β-cell function thus include the sensing of physiological glucose levels, the rapid release of stored insulin vesicles, and the immediate cessation of insulin secretion once glucose levels have been normalized. This complex process requires optimal coordination of multiple regulatory processes that exist in endogenous β cells. Extensive efforts need to be undertaken to ensure that the same regulatory mechanisms are intact in stem cell-derived insulin-producing cells to prevent unwanted complications stemming from hypoglycemia caused by inappropriate or prolonged insulin release. Only when we have thoroughly convinced ourselves that the stem cell-derived cells are the true equivalent of the endogenous β cells should transplantation into human diabetic patients become a reality.